If you're not installing radiant heat over a slab, can you, perhaps, adequately reinforce support in the basement/crawlspace and maybe securely fasten 2x4s to the floor joists and lay 3/4" plywood, as a base and support, over the top of which you can inbed the copper heating pipes in cement/concrete to a finish floor level??

Whoops, sorry for the second reply. I should have caught up on all the posts first.

The answer is, yes, I guess you can. But I have to wonder "why would you want to?" That seems like a long way to go to accomplish what you can easily accomplish without putting in reinforcing beams and cement/concrete just so you can imbed the pipes in it.

If you do a Google on "Radiant Heat Striping" you'll see lots of systems that claim to eliminate this problem, which tells you it is a problem, especially with "embedded pipe systems". (There are others, like the hotter temperatures necessary to keep all that concrete hot, the moisture levels and construction times necessary for curing, and so on.)

Now I've only had the system running for a few weeks, so there may be other problems down the line with the "air heat" method that I'm unaware of, but so far it's working very nicely.

I found that with an ambient room heat of 70, the 90-degree heat pulled by the auto-circ pump raised the floor heat to about 75. Once I closed the loop, changed the pump and installed the hot water heater, each 10 degree rise in water temperature gave me about 3-4 degrees more floor heat. There are some variations in floor heat, 3 degrees plus or minus, depending on the specific area, but it's nothing you can notice with your hand (foot), I can tell only because I am a nerd and spent a couple days moving my digital thermometers (2 of them) around to see how it all played out.